The invention relates generally to communications and, more particularly, to a system and method for coordinating between a cordless fixed base set and a cordless portable handset of a digital cordless telephone the particular radio frequency channel over which the base set and the handset communicate.
In conventional cordless communications systems, such as a digital cordless telephone, a basic communication device includes a cordless fixed part, such as a base set device, and a cordless portable part, such as a handset device. The fixed part has a direct wire link to a standard telecommunications network. The portable part is mobile in relation to the fixed part and does not have any wire link with the fixed part or the network. The fixed part and the portable part communicate through radio frequency (RF) signals transmitted and received by the parts. Communications signals are transmitted over RF by the portable part for receipt by the fixed part, and the fixed part delivers the communications signals over the wire link to the standard communications network. Communications signals received over the wire link by the fixed part from the standard communications network are, likewise, transmitted by the fixed part over RF for receipt by the portable part.
In order to achieve desired communications between the fixed part and the portable part over RF, the fixed part and the portable part must each transmit over a particular RF bandwidth, or "channel", that is receivable by the other of the parts. The particular channel (or channels, as the case may be) authorized for such transmissions may be designated by a particular industry standard, such as the Cordless Telephony Generation 2 (CT2) or the Digital European Cordless Teleconmmunications (DECT) standard, by governmental requirements, such as the U.S. governmnent's designation of the so-called 900 MHz channel, i.e., 902 MHz to 928 MHz (i.e., the "ISM band"), for cordless telecommunications in the U.S., or by some other source. In any event, only particular ones of the authorized channels may be open at any instant to accommodate desired transmissions and receptions between any given fixed part and portable part. This is the case for a variety of reasons, including that other communications devices may also be transmitting and receiving over the same particular channel, transmissions by other devices on neighboring channels may "bleed-over" to the particular channel, noise may be experienced on the particular channel, or the particular channel may otherwise be unsuitable or unavailable.
In conventional cordless communications systems, either the fixed part or the portable part chooses the particular communications channel for RF communications between the parts from among the authorized and open channels. The part that chooses the channel is referred to here as the "master", and the other part is referred to as the "slave". In conventional operations, the slave part has typically employed a phase lock loop of the part to lock-on the communications channel chosen by the master part. Communications over the chosen channel are thereby coordinated based on the choice of the master part and the locking-on by the slave part. The typical master part and slave part have required various filters to distinguish receptions and to provide transmissions over the particular channel chosen by the master part.
Although ideally the fixed part and the portable part choose and lock-on the appropriate channel (or channels) for communications, one problem has been that the parts, in actual operations, sometimes end up communicating over different channels, such as two different adjacent RF channels. This can happen for a number of reasons, including the presence of noise and interference, the quality of the equipment of the fixed part and the portable part, and others. When the parts are communicating, albeit over different channels, transmissions over one channel may "bleed over" into another channel so that the parts believe they are communicating over the same channel when, in fact, they are not. The filters and phase lock loops of the parts are intended to distinguish and choose the appropriate RF channel, however, the parts do not always do so presenting the problem of communications over different channels.
This problem of communications over different channels by the fixed part and portable part is particularly prevalent in digital cordless telephones intended for individuals as the consumers. In the case of those telephones, a major concern has been maintaining low cost of the telephones to the consumers. In designing digital cordless telephones for individual consumers, therefore, lower cost components are generally preferred. There is generally a trade-off, however, between component equipment cost, on the one hand, and quality, on the other hand. Typically, as component equipment cost decreases, so does quality, and vice versa. An example of components available at varied cost with directly proportional varying quality is RF filters. If low cost RF filters are included in the fixed part and the portable part of a digital cordless telephone, a consequence may be that the fixed part and the portable part do not always communicate over the appropriate channel.
Therefore, what is needed is a low cost, accurate, and otherwise desirable system and method for coordinating communication channels (both transmit and receive channels) of the fixed part and the portable part of a digital cordless telephone to achieve communications over a desired RF channel or channels.